Thermally compensated laser

ABSTRACT

The invention is a laser in which an undesired thermal lensing effect of the active laser material is compensated by an addition of a material having a thermal lensing characteristic opposite that characteristic of the laser material. The active laser element and the compensating element are disposed within a regenerative cavity in a regenerative path defined between two end reflectors. As the active laser element is excited by pump energy, a thermal gradient in the laser material results from the excitation and a similar thermal gradient also results in the compensating material to negate the aforementioned thermal lensing effect.

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Inventor Appl. No.

Filed Patented Assignee tates atet Harvey V. Winston Los Angeles, Calif.741,682

July 1, 1968 May 4, 1971 Hughes Aircraft Company Culver City, Calif.

THERMALLY COMPENSATED LASER 5 Claims, 5 Drawing Figs.

US. Cl 331/945,

350/160, 350/175 Int. Cl HOls 3/02 Field of Search 331/945;

350/160, 175 (GNL) [5 6] References Cited OTHER REFERENCES Akhmanov, etal., Thermal Self-Actions of Laser Beams, IEEE J. Quant. Elect, Vol.QE-4, No. 10 (Oct. 1968) pp. 568- 575 Primary Examiner-Ronald L. WibertAssistant Examiner-William L. Sikes Attorneys-James K. Haskell and JohnHoltrichter, Jr.

disposed within a regenerative cavity in a regenerative path definedbetween two end reflectors. As the active laser element is excited bypump energy, a thermal gradient in the laser material results from theexcitation and a similar thermal gradient also results in thecompensating material to negate the aforementioned thermal lensingeffect.

These thermal lensing effects are caused by the formation of a radiusdependent function of temperature, and as the index of refraction formany materials used as active laser elements is a function oftemperature, the overall effect is the formation of a convex or concavelens.

Prior art techniques designed to overcome this problem consisted of thegrinding of a concave or convex lens on an end surface of the laserelement to compensate for the thennal lensing. It should be quiteevident that this method is beneficial at only one pumping level becausethe focal length of the lens is calculated with respect to a particularpumping level. More details on the prior art are provided in an articleby Osterink and Foster, Thermal Effects and Transverse Mode Control in aNdzYAG Lesser," Applied Physics Letters, Vol. 12, l968,p. I28.

Contrary to the prior art, the invention does not incorporate acompensating lensing apparatus which has been ground according to theparticular pumping level'it will experience, but it utilizes a materialwhose thermal lensing characteristics with respect to temperature areopposite in sign to that of the material. These two materials in seriesact as an optically flat lens system at all pumping levels.

It is therefore an object of this invention to provide a laser in whichthe thermal lensing effect is compensated at all pumping levels.

It is another object of the invention to provide a laser in which thevolume of a mode is increased thereby increasing the power output atthat mode.

' It is still another object of the invention to provide a method tofabricate a compensator for the thermal lensing in a laser.

A further object is to provide a laser in which an apparatus forcompensating more accurately the thermal lensing effect can beincorporated in the laser.

Other objects and many of the attendant advantages of this inventionwill be more readily appreciated as the same becomes better understoodby reference to the following detailed description and considered inconnection with the accompanying drawings in which like referencesymbols designate like parts throughout the FIGS. thereof and wherein:

FIG. 1 is a schematic drawing of one embodiment of the invention whichprovides thermal compensation of lensing effect for a laser;

FIG. 2A is a schematic drawing of a laser in which there is no thermallensing;

FIG. 2B is a schematic drawing of a laser in which there is thermallensing;

FIG. 2C is a schematic drawing of a laser in which thermal lensing hasbeen compensated in accordance with the teachings of the invention; and

FIG. 3 is a schematic drawing of another embodiment of the inventionwhich, in addition to compensating for the thermal lensing efiect in thelaser as shown in FIG. 1, provides an apparatus for fine tuning of thecompensator.

With reference to FIG. 1, there is shown a thermally compensated laser11 generally comprising an active laser element 13 pumped by aconventional pumping source 15 and placedbetween a pair of opticallyreflecting surfaces 17 and 19 which form the ends of a resonant cavity.Also positioned within the resonant cavity is a thermal compensatorelement 21 for compensating for the thermal lensing effects of theactive element 13 when it is pumped by the source 15. The end surfaces23 and 25 of the laser element 13 and the end surfaces 27 and 29 of thethermal compensator element 21 may be coated with a conventionalantireflection coating to improve the efficiency of the laser 11 and anoutput means such as having one or both of the reflecting surfaces 17and 19 partially transmissive.

The active element 13 may comprise any known substance which exhibitlasing action to provide coherent light along the regenerative pathprovided between the reflecting surfaces making up the resonant cavity.This active material may comprise a solid, e.g. a ruby crystal or a Nd*:YAG crystal. Also,

the pumping source 15 may comprise any source of energy which is capableof exciting the molecules or ions in the active element 13 to a lasingstate and need not be an optical pump source as indicated in the drawingbut can be any other type of pump source suitable for exciting theparticular laser material used. In other words, the pumping source 15 isa source of energy which is capable of establishing the necessaryinverted population density condition in the active element 13. Examplesof such pumping sources are means for-providing a radio frequency field,a xenon flash tube or any other suitable type of energy source.

Experimentally, it has been found that the index of refraction changesproportionally to the change in temperature and according an equationcan be written n=n -lu(T-T,,) with n, being the index at temperature T,and a being the derivative of the index with respect to temperature.

The function of temperature is radius dependent because, while the laserelement 13 is uniformly heated by the illumination from the pumpingsource 15, it can only be cooled at its surface. Thus, a first orderapproximation of the function of temperature may be expressed by aparabolic equation with B and 7 being constants depending on thematerial, environmental conditions, and illumination.

It can also be deduced from the above information that the change in theindex of refraction is a radius'dependent function which can beexpressed as Thermal lensing occurs when the derivative of thisfunction, An(rr, is not zero. A negative derivative indicates a convexlens, while a positive derivative indicates a concave lens.

In order to obtain the desired results, the compensating element 21 maybe doped with known impurities to produce an absorption spectrum similarto that of active laser element. Of course, it is assumed that bothelements 13 and 21 are exposed to the same heating condition at eachwavelength. If not, then a different absorption spectrum should beobtained. As the derivative of the index of refraction for thecompensating element is chosen opposite in sign to that of the activelaser element (although not necessarily equal in magnitude), acombination of this element having an appropriate length in series withthe active laser element will negate the thermal lensing effect at allpumping levels.

The appropriate length is determined by the requirement that there mustbe no unequal change in the path length, AL, with respect to the radius,r. The path length L, is given by the equation L,,=L,,,+L,(nr-l )+L (n-l where I. is the spacing between the reflectors l7 and 19, L, is thelength of the active laser element, L is the length of compensatingelement, n, is the index of refraction of active laser element 13, and nis the index of refraction of the compensating element 21. The change inpath length is determined by the equation Since for both elements thechange in index of refraction is a product of the derivative of theindex and the change in temperature gradient, then the appropriatelength of the compensating element is given by the equation c l/ c) l-The derivatives of the index of refraction, a, for various materials arelisted below:

Material at l0 Sapphire (7065A.) +l2.6

Polystyrene (7674A.) -200 NaC1(60C.,l.1 J.) 36.4

- with respect to the radius, r, there is no lensing effect. However, auniform change in path length creates problems in staschemes.

cal mode as if there were no thermal lensing; FIG. 2B shows the volume33 of the same mode as if there were thermal lensing; and in FIG. 2C thevolume 35 of the same mode is shown increased over that volume shown inFIG. 28 after thermal lensing has been compensated. For FIGS. 2A, 2B and2C, it is assumed that all other conditions are the same. Of course, itis realized that a series of tradeofis on all these conditions wouldconstitute engineering improvements of a laser. For example, fiatreflectors l7 and 19 are shown, although often it is a better design tohave spherical reflectors.

In FIG. 2B the thermal lens formed is equivalent to a thick convex lens,and a parallel laser beam 37 is caused to converge inside the laserelement 113 to thus decrease the volume of the modes within theregenerative path. in H0. 2C, the thermal lens is compensated by thecompensating element 21 which acts as a thick concave lens and causesthe converging laser beam 39 to diverge enough so that it is againparallel. The volume of the mode within the regenerative path is thusincrease to approximately its original size.

A thermally compensated laser 41 shown in FM]. 3 includes the sameelements as those shown in FIG. l with the exception that a fine tuningapparatus 43 has been added. The fine tuning apparatus consists of anadditional illumination source which may be varied in intensity tomaintain the function of temperature in the compensating elementapproximately equal to that in the active laser element.

From the foregoing, it can be seen that there has been described a laserin which compensation of the thermal lensing effect at all pumpinglevels is accomplished. It has also been shown that mode volume andpower output of that mode is increased by compensating for the thermallensing elTect;

Furthermore, a method of fabricating a thermal compensator for thethermal lensing effect has been described. Additionally, an apparatusfor finely tuning the compensator has been discussed.

Although specific embodiments have been herein 'described, it will beappreciated that other organizations of the specific arrangement shownmay be made within the spirit and scope of the invention. For example,the active laser The thennal lensing effect and the compensation of thatef- I feet for the laser 11 shown in FIG. 1 are diagrammed in FlGS.

1A, 2B, and 2C. FIG. 2A shows the volume 31 of a hypothetimaterial maybe any material exhibiting laser action, and the spacing of the variouselements of the thermally compensated lasers in FlGS. l and 2 may beother than that shown. F urthermore, it should be noted that thesketches in various FlGS. are not drawn to scale and that the distancesof and between various FlGS. are not to be considered significant. Asset forth previously, other components similar in function may besubstituted for the components shown in the drawings.

Accordingly, it is intended that the foregoing disclosure and theshowings made in the drawings shall be considered only as illustrationsof the principles of this invention and are not to be construed in alimiting sense.

lclaim: l. A thermally compensated laser, comprising: a laserregenerative cavity including end reflectors defining a regenerativepath therebetween and including means for providing a laser output beam;an active laser material disposed within said laser cavity in saidregenerative path and having a certain lensing characteristic withrespect to temperature; pumping means coupled to said laser material forproducing pump energy to excite said laser material to a lasing state;and a second material disposed within said laser cavity in saidregenerative path exposed to said pump energy and having a lensingcharacteristic with respect to temperature opposite in direction to saidcharacteristic of said laser material. 2. A thermally compensated laseraccording to claim 1,

wherein said second material has been doped with impurities to obtain anabsorption spectrum equal to that of said active laser material.

3. A thermally compensated laser according to claim 2, wherein thelength of said second material is determined by the product of the ratio(-oq/oa) and the length of said active material.

4. A thermally compensated laser according to 'claim 1, wherein meanscoupled to said second material for maintaining a temperaturedistribution in said second material approximately equal to atemperature distribution in said active laser material are provided.

5. A thermally compensated laser according to claim 3, wherein saidactive laser material is Nd :YAG; and said second material ispolystyrene.

(2 33 UNITED STATES PA'II'IN'I OFFICE CERTIFICATE OI? CORRECTION PatentNo. 3 577 I 098 y 4 I 1 Dated lnventofls) Harvey V. Winston It iscertified that error appears in the above-identified pater and that saidLetters Patent are hereby corrected as shown below:

W01. 1, line 16 "Lasser" should be .Laser.

Col. 2, line 14 "according" should be accordingly-.

line 32 delete "An(rr, and substitute therefor An (r)., with respect tothe radius r,--. Col. 3 line 5, "1A" should be --2A-.

line 25, "increase" should be -increased.

Signed and sealed this 26th day of October 1971 (SEAL) Attest:

EDWARD M. FLETCHER ,JR. ROBERT GOTTSCHALK Attesting Officer ActingCommissioner of Pa

2. A thermally compensated laser according to claim 1, wherein saidsecond material has been doped with impurities to obtain an absorptionspectrum equal to that of said active laser material.
 3. A thermallycompensated laser according to claim 2, wherein the length of saidsecond material is determined by the product of the ratio (- Alpha l/Alpha c) and the length of said active material.
 4. A thermallycompensated laser according to claim 1, wherein means coupled to saidsecond material for maintaining a temperature distribution in saidsecond material approximately equal to a temperature distribution insaid active laser material are provided.
 5. A thermally compensatedlaser according to claim 3, wherein said active laser material is Nd:YAG; and said second material is polystyrene.